Chastity overseas Credence and Mr. Graves' alleyway interaction or maybe just heading to alleyway, or realizing her usual spot is taken by some men and no one other then Credence. It leads to her misunderstanding of the nature of their interactions (no active Gredence at that moment).

And so Chastity is bewildered, that Credence had the guts to do it at all, and so very jealous that he managed to score such a fine and obviously rich man.
So she begins to slander him to Mary Lou (through not about what she thought she had seen), spies on him both in and out of church and at the same time tries to find out anything about Mr. Too-Good-For-Credence, while making plans to seduce him and steal him for herself.

+ pre-movie and no Grindelwald
++ Chastity occasionally doing favors for older gentlemen at the Alleyway (she wants some money, or tries to catch some big fish, maybe it's a riot against Mary Lou or something else altogether)
+++ Chastity's thoughts and feelings both before and after finding out the 'truth' about Credence
+++++ Chastity making a move on Mr. Graves